For many years concrete roadways and other concrete slabs have incorporated reinforcing steel laid out in a rectangular lattice pattern for the purpose of adding strength to the concrete in order to resist slab failure when cracks develop in the concrete due to normal environmental conditions and heavy use. When cracks in the concrete develop, the mat lattice structure of the reinforcing bars perform the function of holding the concrete together at the point of the cracks, preventing separation of the concrete and failure of the slab. In order for the reinforcing bars to effectively perform this function, however, engineers have determined that the rebar mat should be located at the center of the poured slab. The normal preferred location is in the center of the slab, referred to as Thickness/2 or t/2. The rebar mat must also retain its designed rectangular lattice shape in order for the longitudinal and transverse rebar sections to provide the maximum reinforcing strength at anticipated cracking areas, and in order for the rebar ends of each section to line up with the rebar ends of adjoining sections of rebar mat as additional lanes are constructed. For many years contractors and public road building engineers have been aware that current methods for building the rebar mat and elevating the mat off the surface of the roadbed or slab often fail to maintain the rebar mat in a rectangular shape and in the desired location within the slab after the placement of concrete which results in premature failure of the concrete road or slab.
Currently, rebar mats used in road construction are built using a three step method: (1) laborers lay out rebar sections on the roadbed in the location where the rebar mat section is to be assembled forming a template with the transverse bars spaced properly and usually 3, 4 or 5 longitudinal bars laid out on top of the transverse bars, and tied with wire at the rebar intersections to form the template; (2) once the template has been laid out and tied together with wire, the rebar template is lifted off the surface of the roadbed and devices, known as "rebar chairs, " are placed under each transverse bar in the rebar template, with an adequate number of rebar chairs being used to elevate the rebar mat to the desired height and then setting the rebar on top of the rebar chairs, usually in a slot built into the chair; and (3) placing the remaining longitudinal rebar sections on top of the elevated template, and wire tying them at alternating intersections to complete construction of the rebar mat.
This traditional method for building rebar mats is deficient in two major respects. First, the rebar chairs are held upright by the base of the chair sitting on the roadbed and are not firmly locked to the rebar mat sections. When lateral forces, common on the job site, are exerted against the rebar mat, the chairs can rotate off of or around the rebar mat section and the entire rebar mat can fall to the roadbed. This situation can also occur during the pouring of the concrete when the lateral forces exerted by the in-flowing concrete can wash the rebar chairs out from under the rebar mat causing the mat within the slab to sag to the ground where the chairs have failed after the concrete is poured. Second, the wire tied rebar intersections provide no strength against angular deflection of the rebar, known as racking, when extreme forces of the concrete placement and the paving vibrator are applied. This method of mat construction can result in a rebar mat that is deformed into a parallelogram shape, that has less effective reinforcing capacity, and that causes the misalignment of rebar ends for attaching to the next adjoining section of rebar mat. These problems have been exacerbated in recent years with the growing use of less viscous, quick drying concrete mixtures.
Since this is such a difficult problem, there have been a number of attempted solutions. One solution known to the inventor is a device described in U.S. Pat. No. 3,378,981 that includes a generally rectangular box-like support made of sheet metal that has first and second vertically extending walls with each wall including a recess to receive rebar. Tabs stamped into the metal shape can be bent over to hold the rebar in place. The difficulty with this device is that the rebar connector is metal, which by being in contact with the road bed, forms a path for corrosion to travel to the rebar mat and greatly accelerates the corrosion of the entire rebar mat.
Oxidation (formed by corrosion) of the rebar mat produces internal expansion forces that will cause the entire slab to crack and fail.
Consequently, these metal chairs are unacceptable for use in today's road construction. Moreover, the basic design of this generally rectangular box-like connector results in a space or pocket being formed by the intersection of three planes which will completely fill with concrete during concrete placement. This produces a void or hollow space in the concrete roadbed. This "voiding" problem has been exacerbated today because of the use of "stiffer" concretes, that are poured with a viscosity higher than in previous years. Consequently, the propensity for forming "voids" in the concrete prevent the use of these rectangular metal chairs in any publicly funded roadway and highway construction. In addition, because the chairs are constructed of metal they offer no resistance to racking forces. The metal chairs simply bend to accommodate to the deformed shape of a racked rebar mat.
The most popular rebar chair in use today is a plastic "tee-pee" shaped chair having a triangular shape, that is wider at the bottom and narrows to a point at the top where a U-shaped saddle acts as a receiver for rebar placed on top of the chair. Often the U-shaped saddle section is formed so as to snap around a portion of the rebar when it is lowered into the saddle section. The tee-pee chairs are made of various types of plastic, selected primarily upon manufacturing costs considerations. The design of this chair provides no means to prevent the chair from rotating off the rebar when lateral forces are applied to the mat, either before or during the concrete pour. Furthermore, these tee-pee shaped chairs offer no resistance to the racking forces applied to the rebar mat during the concrete pour.
Another similar type of rebar chair in commerce usage is referred to as the "square base chair." In function, it is identical to the "tee-pee" chair in that it is simply placed under the rebar mat, and snaps to a single section of bar. The larger base of this chair provides some additional strength, but offers no significant anti-rotation or anti-racking capabilities. Furthermore, both the "tee-pee" chair and the "square base chair" present significant voiding potential at the chair base at the intersection of the three vertical planes which form the support for the rebar mat.
Another type of chair used in some locations, referred as the "high-chair," has four round vertical plastic legs connect at the top with an "X" pattern to form a cradle for the rebar, and depending on the chair height, a second "X" between the legs in the lower part of the chair for strength. This rebar chair eliminates the voiding potential present in the "tee-pee and square base chairs; but again provides no anti-rotation or anti-racking forces. Furthermore, the "high-chair" must be tied to the mat itself adding an additional labor procedure to complete the mat.
The practice of tying the rebar intersections with wire somewhat mitigates against the transverse rebar sections from moving out of the designed location during the concrete pour, although this effect is not entirely eliminated. The wire tied joints, however, provide no support at the rebar intersections for resisting the forces that tend to deform the entire mat into a parallelogram. Thus, racking of the rebar mat remains a constant problem with today's less viscous concrete mixes used in roadway and other slab-type construction.
A primary concern for the contractor is that the current method of constructing rebar mats is extremely labor intensive. As discussed above, three separate steps are required to be performed by laborers on the location of the mat building. These steps include (1) laying out the rebar mat template, and tying it together with wire, (2) installing the chairs under the rebar mat template, in sufficient quantities to hold the mat off the roadbed, and (3) placing the remaining rebar sections on the template, after it is elevated off the roadbed, and making a second trip down the rebar mat to wire tie the rebar intersections together.
The present invention overcomes these problems by providing chairs that will not rotate off the rebar when lateral forces to the rebar mat are encountered. This anti-rotation feature results from the unique locking mechanism in the chair base and cap which uses the rebar mat itself to hold the chair in an upright position. Once the chair is locked on to the rebar mat at selected intersections, the rebar in the mat serves as a back-lock to prevent the cap from disengaging from the chair as rotational forces are applied. This feature of the present invention eliminates chair rotation. All other chairs use the chair base to retain the rebar support device in the vertical, upright, position. Consequently, all current chairs in use will rotate over when lateral forces are applied.
In addition, the inventive chairs eliminate wire tying the rebar mat template together, and permit the first and second steps of building and elevating the rebar template off the ground to be performed in a single step, which results in decreased labor costs. With the present invention, the rebar template sections are fastened together as the chairs are placed under the template to elevate the mat off the road bed. This feature eliminates over 50% of the required wire tie joints, and combines two steps of mat assembly into a single step.
The chairs of the present invention also provide substantial anti-racking support at the rebar intersections. After all the chairs are affixed to the rebar mat, they work together as a unit to prevent the mat from being deformed out of square.
The chairs and locking caps of the subject invention, when used as a system, virtually eliminate the possibility of mat failure during the concrete pour, either due to collapse of the mat to the road bed or from racking. The system of the subject invention also substantially lowers the amount of labor required to build the rebar mat by allowing the mat building process to be completed in a single trip down the mat.
The invention also incorporates an apparatus relating to the construction of multiple layer rebar mats that are in common usage in high traffic volume highways, particularly in urban areas. Current methods for building double and triple layer rebar mats involve simply building multiple mats on top of one another, using increasingly higher chairs for the upper level mats. This method retains all of the disadvantages of current rebar mat construction described above, and is further subjected to two additional and more serious problems. First, as the chairs become increasing higher in length, their propensity to rotate or tip over increases. Thus, the potential of upper layers of rebar mat falling onto the lower layers is always present, in spite of the use of expensive high rise chairs purportedly designed to mitigate against this problem.
Second, specifications for multiple layer mats require that the rebar sections be aligned along the same vertical planes. The only chair known to the inventors at this time which will allow the proper alignment of the upper layer mat, is a "spider-leg" chair, with four metal legs that allow it to be placed on top of the lower mat rebar intersection. Because this chair design requires that the chair be made of metal, a rust path to the upper rebar mats results, which is unacceptable for the same reasons described above regarding other metal chairs. When plastic chairs are used, they must be installed adjacent to the lower rebar sections, thus preventing the alignment of the upper mat bars with the lower mat bars. Thus, neither above described chair meets the accepted specifications for building multiple layer rebar mats.
The chair of the present invention, can include a special locking section which allows multiple rebar mats to be constructed on the same chair base. This assures the uniform separation of the multiple mats as designed by the project engineers and assures proper alignment of the bars in the lower and upper mats. Presently, there is no known rebar chair that allows for the use of a single chair to construct and lock multiple layer rebar mats together at the predetermined height.
The chairs of the present invention are also designed so as to virtually eliminate the phenomenon of concrete voiding, which is commonplace with all types of today's plastic chairs, with the exception of the "high-chairs". The chair legs of the present invention tapper to the road bed or their base, greatly reducing the size of the three plane intersections below the rebar mat. This results in minimal voiding potential. As discussed above, concrete voiding occurs at the intersection of the three planes that are designed into the base of all plastic chairs for vertical support. Because the chairs must be located beneath the rebar mat, the vibrators used in the concrete placement cannot exert direct forces on the concrete slurry below the mat. This results in air pockets forming in the chairs corners where the three planes (two vertical and one horizontal) intersect, creating voids in the base of the slab. Where voids are sufficiently numerous, the upward support force of the concrete is no longer uniform, which over time increases the likelihood of slab failure. Concrete cracking tends to run toward the voided areas.
The locking cap of the chairs of the present invention also provides another significant advantage in the economics of rebar mat construction for roadways and other slabs. The rebar mats of the present invention, once assembled, provide sufficient anti-racking force to the entire mat allowing the mats to be lifted and moved as a single unit. This allows for offsite construction of the mats and transportation to the job site as needed. This feature allows for substantial savings in the cost of rebar mat building and for faster completion of roadbed construction or other slab construction, resulting in as much as an 80% decrease in on-site time for laying the rebar mats. This offsite construction is particularly suited for tilt wall and retaining wall applications, where the rebar mats are small enough to allow transportation as a single unit.
Lastly, the inventive chairs are particularly well suited for use in tilt wall construction, where concrete slabs are poured on the ground over a rebar mat, and then tilted up into place. In tilt wall applications, the base of the chair will be visible on the underside of the wall after it has tilted upright. Because the current invention uses the rebar mat to hold it upright, rather than a large base, the support leg can be tapered to a point at the base, virtually eliminating any chair footprint after the tilt wall section has been tilted into the vertical position. Additionally, the chairs and locking caps of the present invention also include features that allow for the construction of rebar mats for vertical slabs and the construction of rebar mats on sloping surfaces.